Fiber reinforced composites consisting of reinforcing fibers and a matrix resin are widely used for sporting goods such as golf shafts, fishing rods, tennis rackets and the like. Such composites are also used in the aerospace industry and other general industries since they are light in weight and excellent in mechanical properties.
Fiber reinforced composites are produced by various methods and, at present, it is widely practiced to produce them by using sheet-like intermediate materials called prepregs in which reinforcing fibers are impregnated with a matrix resin. According to these methods, a plurality of prepreg sheets are laminated and heated to form a fiber reinforced composite.
Matrix resins used for prepregs include both thermosetting resins and thermoplastic resins, but in most cases, thermosetting resins are used. Epoxy resins (cured) are mainly used since they have excellent mechanical and chemical properties such as heat resistance, stiffness, dimensional stability and chemicals resistance. The commonly used terms "thermosetting resin" and "epoxy resin" include two cases: 1) a prepolymer and 2) a cured product obtained by reacting a composition containing the prepolymer and other ingredients. In the present specification, the terms "thermosetting resin" and "epoxy resin" are used to mean a "prepolymer" unless otherwise stated.
Problems often encountered in the use of a prepreg obtained by using a thermosetting resin are the tackiness between overlaid prepreg sheets and the drapability of the prepreg. These properties greatly affect prepreg handling convenience.
Overlaid sheets of the prepreg are likely to be delaminated if the tackiness of a prepreg is too small. On the contrary, it is difficult to separate the sheets of the prepreg once overlaid by mistake if the tackiness of a prepreg is too large. The working efficiency in laminating sheets of a prepreg on a mold or mandrel with a curved surface declines remarkably if the drapability of a prepreg is poor.
In recent years, sporting goods such as golf shafts and fishing rods are remarkably reduced in weight, and prepregs suitable for light-weight design are being demanded. Prepregs using high modulus fibers, especially high modulus carbon fibers as reinforcing fibers have been demanded in recent years since they facilitate lighter-weight design. Furthermore, the demand for prepregs with a high reinforcing fiber content is also growing.
However, prepregs decline in drapability if high modulus carbon fibers are used as reinforcing fibers. Furthermore, tackiness tends to be lower if the reinforcing fiber content is higher since the amount of the resin distributed at the surface of the prepreg becomes smaller. Thus, conventional prepregs using a matrix resin have the problem that tackiness and/or drapability becomes insufficient.
Several proposals have been made to improve resin compositions for obtaining good tackiness or drapability, as described later. However, these techniques generally have the problem that tackiness can only be improved at the expense of drapability.
Golf shafts and fishing rods are formed by winding prepregs around a mandrel with a relatively small diameter. If the force to delaminate a prepreg laminate exceeds the tackiness between the laminated sheets of the prepreg, the prepreg wound around a mandrel is delaminated, which disturbs the winding work. The force to delaminate the prepreg is larger if the drapability is smaller. Therefore, even if tackiness is improved at the sacrifice of drapability, the mandrel winding work itself cannot be improved significantly.
Known methods to improve the tackiness of a prepreg include adding a polymer such as a thermoplastic resin or an elastomer to an epoxy resin. Methods of adding a polymer to an epoxy resin include methods of adding a polyvinyl formal resin as stated in JP-A-58-8724 an JP-A-62-169829, methods of adding a polyvinyl acetal resin as stated in JP-A-55-27342, JP-A-55-108443 and JP-A-6-166756, a method of adding a polyester polyurethane as stated in JP-A-5-117423, a method of adding a poly(meth)acrylate polymer as stated in JP-A-54-99161, a method of adding a polyvinyl ether as stated in JP-A4-130156, a method of adding a nitrile rubber as stated in JP-A-2-20546, etc.